1. Field of the Invention
The invention relates generally to phosphopantetheinyl transferases that are involved in the activation of a polyketide synthase to synthesize long chain polyunsaturated fatty acids (such as docosahexaenoic acid and eicosapentaenoic acid.
2. Description of the Related Art
The primary products of fatty acid biosynthesis in most organisms are 16- and 18-carbon compounds. The relative ratio of chain lengths and degree of unsaturation of these fatty acids vary widely among species. Mammals, for example, produce primarily saturated and monounsaturated fatty acids, while most higher plants produce fatty acids with one, two, or three double bonds, the latter two comprising polyunsaturated fatty acids (PUFA's). Very long chain PUFAs such docosahexaenoic acid (DHA, 22:6) and eicosapentaenoic acid (EPA, 20:5) have been reported from several species of marine bacteria, including Moritella (Vibrio) marina and Shewanella sp. (U.S. Pat. No. 6,140,486) and from marine algae such as Schizochytrium sp. and Thraustochytrium sp. (US Patent Publication 20040235127).
Two main families of PUFAs are the omega-3 fatty acids (also represented as “n-3” fatty acids), exemplified by docosahexaenoic acid and the omega-6 fatty acids (also represented as “n-6” fatty acids), exemplified by arachidonic acid (ARA, 20:4). PUFAs are important components of the plasma membrane of the cell and adipose tissue, where they may be found in phospholipids and triglycerides, respectively. PUFAs are necessary for proper development in mammals, particularly in the developing infant brain, and for tissue formation and repair.
Several disorders respond to treatment with PUFAs. Supplementation with PUFAs has been shown to reduce the rate of restenosis after angioplasty. The health benefits of certain dietary omega-3 fatty acids for cardiovascular disease and rheumatoid arthritis also have been well documented (Simopoulos, 1997; James et al., 2000). Further, PUFAs have been suggested for use in treatments for asthma and psoriasis. Evidence indicates that PUFAs may be involved in calcium metabolism, suggesting that PUFAs may be useful in the treatment or prevention of osteoporosis and of kidney or urinary tract stones.
The majority of evidence for health benefits applies to the long chain omega-3 fats, EPA and DHA, which are in fish and fish oil. With this base of evidence, health authorities and nutritionists in Canada (Scientific Review Committee, 1990, Nutrition Recommendations, Minister of National Health and Welfare, Canada, Ottowa), Europe (de Deckerer et al, 1998), the United Kingdom (The British Nutrition Foundation, 1992, Unsaturated fatty-acids—nutritional and physiological significance: The report of the British Nutrition Foundation's Task Force, Chapman and Hall, London), and the United States (Simopoulos et al., 1999) have recommended increased dietary consumption of these PUFAs.
Major long chain PUFAs of importance include DHA and EPA, which are primarily found in different types of fish oil, and ARA, found in filamentous fungi such as Mortierella. For DHA, a number of sources exist for commercial production including a variety of marine organisms, oils obtained from cold water marine fish, and egg yolk fractions. However, there are several disadvantages associated with commercial production of PUFAs from natural sources. Natural sources of PUFAs, such as animals and fungi, tend to have highly heterogeneous oil compositions. The oils obtained from these sources therefore can require extensive purification to separate out one or more desired PUFAs or to produce an oil which is enriched in one or more PUFAs.
Natural sources of PUFAs also are subject to uncontrollable fluctuations in availability. Fish stocks may undergo natural variation or may be depleted by overfishing. In addition, even with overwhelming evidence of their therapeutic benefits, dietary recommendations regarding omega-3 fatty acids are not heeded. Fish oils have unpleasant tastes and odors, which may be impossible to economically separate from the desired product, and can render such products unacceptable as food supplements. Animal oils, and particularly fish oils, can accumulate environmental pollutants. Foods may be enriched with fish oils, but again, such enrichment is problematic because of cost and declining fish stocks worldwide. This problem is also an impediment to consumption and intake of whole fish. Nonetheless, if the health messages to increase fish intake were embraced by communities, there would likely be a problem in meeting demand for fish. Furthermore, there are problems with sustainability of this industry, which relies heavily on wild fish stocks for aquaculture feed (Naylor et al., 2000).
Other natural limitations favor a novel approach for the production of omega-3 fatty acids. Weather and disease can cause fluctuation in yields from fish. Large scale fermentation of organisms such as Mortierella is expensive. Natural animal tissues contain low amounts of ARA and are difficult to process. Microorganisms such as Porphyridium and Mortierella are difficult to cultivate on a commercial scale.
A number of marine microorganisms produce very long-chain PUFAs such as DHA and EPA by a polyketide synthase (PKS) mechanism. PKSs are enzyme complexes composed of multifunctional polypeptides that catalyze the synthesis of complex molecules from simple substrates in an iterative fashion. PKSs are well known in the art and numerous examples of such sequences can be found in the literature. In Moritella marina, a PKS synthesizes DHA from malonyl-CoA and acetyl-CoA. To activate this PKS, a phosphopantetheinyl transferase is required.
Phosphopantetheinyl transferases (Ppts) catalyze the post-translational activation of carrier proteins, fatty acid synthases, polyketide synthases, and non-ribosomal polypeptide synthetases by the covalent attachment of the 4′-phosphopantetheine moiety of coenzyme A to a conserved serine residue, a reaction required for the biosynthesis of natural products including fatty acids, polyketides, and nonribosomal peptides. Ppts have been classified according to their carrier protein specificity. In organisms containing multiple phosphopantetheine-requiring pathways, it has been suggested that each pathway has its own Ppt. While the M. marina PKS has been cloned (U.S. Pat. No. 6,140,486 (Facciotti et al.)), the Ppt was not found. Allen and Bartlett (2002) stated that they were unable to clone a Ppt gene from Moritella. 
A number of approaches have been attempted for production of DHA and EPA in plants (WO05103253A1 (Singh et al), WO04071467A2 (Kinney et al)). These approaches had in common the use of desaturases/elongases in a stepwise fashion. This approach has the disadvantage of using 6-8 genes and leads to the accumulation of intermediates, a potentially undesirable outcome. Using a PKS/Ppt approach, the number of transgenes required would be smaller (4-5) and the accumulation of intermediates is not expected.
Therefore, it would be advantageous to obtain genetic material involved in long-chain PUFA biosynthesis and to express the isolated material in a plant system, in particular, a land-based terrestrial crop plant system, which can be manipulated to provide production of commercial quantities of one or more PUFA's. There is also a need to increase omega-3 fatty acid intake in humans and animals. Thus there is a need to provide a wide range of omega 3-enriched foods and food supplements so that subjects can choose feed, feed ingredients, food and food ingredients which suit their usual dietary habits. Particularly advantageous would be seed oils with increased DHA or EPA.
Currently there is only one omega-3 fatty acid, ALA, available in vegetable oils. However, there is poor conversion of ingested ALA to the longer-chain omega-3 fatty acids such as EPA and DHA. It has been demonstrated in copending U.S. Publication. No. 20040039058 for “Treatment And Prevention Of Inflammatory Disorders,” that elevating ALA intake from the community average of 1 g/day to 14 g/day by use of flaxseed oil only modestly increased plasma phospholipid EPA levels. A 14-fold increase in ALA intake resulted in a 2-fold increase in plasma phospholipid EPA (Manzioris et al., 1994). Thus, to that end, there is a need for efficient and commercially viable production of PUFAs using a polyketide synthesis complex and the Ppts that activate the complex, genes encoding the Ppt, and recombinant methods of producing them. A need also exists for oils containing higher relative proportions of DHA or EPA, and food compositions and supplements containing them. A need also exists for reliable and economical methods of producing specific PUFA's. Oils derived from oilseed crops such as canola, soybean, corn, sunflower, or flax, that express a bacterial PKS complex are enriched in a long chain PUFA, DHA or EPA. Such oils can be utilized to produce foods and food supplements enriched in omega-3 fatty acids and consumption of such foods effectively increases tissue levels of EPA and DHA. Foods and foodstuffs, such as milk, margarine and sausages, all made or prepared with omega-3 enriched oils, will result in therapeutic benefits. Thus, there exists a strong need for novel nucleic acids of phosphopantetheinyl transferases capable of activating PKS for use in transgenic crop plants with oils enriched in PUFAs, as well as the improved oils produced thereby.